This invention relates to a moving target indication system.
Frequency-modulated continuous wave radar systems are known. Such systems normally comprise a variable frequency oscillator the output of which is coupled to an aerial, means for repeatedly sweeping the output frequency of the oscillator between first and second values, a mixer circuit having a first input to which the output of the oscillator is coupled and having a second input to which an aerial is coupled, and a signal processing arrangement having an input to which the output of the mixer circuit is coupled. If the oscillator output signal transmitted by the aerial should be reflected back by a stationary target and fed to the second input of the mixer, and if the sweeps of the oscillator output frequency are linear, the mixer produces an output beat frequency signal (constant IF sweep end effects being neglected) the frequency of which is proportional to the target range. In such systems the signal processing arrangement, for example a multiple filter, separates the mixer output signal into its component frequencies to provide a read-out indicative of the various ranges at which reflecting targets are present. If on the other hand a given target is moving towards or away from the system the respective output frequency of the mixer will gradually decrease or increase respectively. Moreover, if the sweeps of the output frequency of the oscillator are in an upward direction the respective mixer output frequency will in these circumstances be lower or higher, respectively, at any given time than it would be if the target were stationary at the same range, because the frequency of the return signal will be higher or lower, respectively, than it would otherwise be, due to the Doppler effect. Such a system could be arranged to distinguish between a stationary target at a given range and a moving target at a different range by observing the relevant output frequency of the mixer over a period of time which is sufficient to allow the change in frequency with target range to become apparent. However, such a period of time can be considerable, particularly with relatively slowly-moving targets, which makes the use of such a method as the basis of a moving target indicator impracticable.
If, instead of repeatedly sweeping the oscillator frequency in an upward direction, sweeps in a downward direction are employed then, if a target is moving towards or away from the system, the mixer output frequency will, due to the Doppler effect, be higher or lower, respectively, than it would be if the target were stationary at the same range, i.e. it will be shifted in the opposite sense to the shift occurring with sweeps in the upward direction. Thus sweeping the oscillator frequency first in one direction and then in the other and averaging the spectra of the two resulting output signals from the mixer enables the effects of the Doppler shifts to be eliminated (although this becomes increasingly difficult for higher values of Doppler shift, i.e. Doppler shifts which are sufficiently large to become comparable with the output frequency of the mixer which would be obtained for a stationary target at the same range). This fact is known (see, for example, the book "Radar Handbook" edited by M. I. Skolnik, page 16-26 (1970)). If instead of averaging the spectral components lying within each of successive frequency ranges for the two sweeps their differences were calculated, then the existance of a non-zero result would theoretically indicate that the relevant pair of components relates to a moving target. However, if such a non-coherent system, i.e. a system in which only the moduli of the signals in each range cell are considered, not their phases, were used as the basis for a moving target indicator then, because in practical situations there is a great deal of "clutter" in the return signals, the presence of a small moving target very likely would be masked by a large clutter target. In addition such a system would completely fail to detect a moving target if the target did not move into a different range cell between one sweep and the next and there were no clutter in the relevant cell with which to compare it.